Interference between electrical systems is a major source of concern for designers of communications systems, even if the communications system being developed happens to use wires. Although interference and interference avoidance are of the utmost importance in wireless communications systems, wired communications systems using electrical signaling and conductive wires can also receive interference from external sources on the conductive wires. Furthermore, wired communications systems can also transmit interference (unintended radiation) on the conductive wires. Therefore electrical systems operating in the close proximity of communications systems (both wired and wireless) may be interfered with to some extent from these communications systems.
In general, when a communications system is developed that interferes with the operation of existing communications systems and electrical systems, it is the responsibility of the communications system being developed to reduce the interference that it generates. As an example, asymmetric digital subscriber lines (ADSL) (and other DSL systems which use discrete multitone (DMT) modulation) is a technology that provides a high-speed communications system using existing telephone lines. ADSL divides the available bandwidth into a plurality of subchannels, assigns data to subchannels, converts the subchannels into a time-domain signal, and then transmits the time-domain signal over the telephone line. Unfortunately, the frequency band used in most ADSL systems encompasses a significant portion of AM radio and amateur radio spectrum and therefore, AM radios and amateur radio systems operating in the vicinity of ADSL transmission lines may receive interference. Since AM radio and amateur radio spectrums are protected, it is up to designers of ADSL systems to prevent their communications systems from interfering with these devices.
A general idea behind several different method for reducing interference from communications systems is to place a limit on the transmit power on portions of the transmission that is causing interference. Essentially, the frequencies that are causing interference are notched out of the transmission. One method for doing this involves the use of notch filters (such as finite impulse response (FIR) or infinite impulse response (IIR) filters). A second method, which is well suited for use in ADSL systems, simply turns off subchannels which correspond to the interfering frequencies.
More advanced methods that exploit properties of the ADSL signal can reduce the number of subchannels that need to be turned off. By reducing the number of subchannels, the impact of the interference reduction techniques on the data rate of the communications system can be reduced. One method, referred to as transmitter windowing, applies a window to the transmitted signal. A second method uses subchannels (referred to as dummy subchannels) on either side of the subchannels corresponding to the interfering frequencies to transmit dummy signals (as opposed to data) to improve the notch. When properly designed, transmitter windowing and/or dummy subchannels can reduce the percentage of the transmission bandwidth that is dedicated to interference reduction.
One disadvantage of the prior art is that the use of zeroes in the frequency ranges that are causing the interference can consume a significant percentage of the overall transmission bandwidth. This can reduce the data rate of the communications system.
A second disadvantage of the prior art is that the use of filters and transmitter windowing to eliminate the frequency ranges that are causing interference can make it difficult for the receiver of the signal to equalize the received signal.
A third disadvantage of the prior art is that the use of calculated values (dummy signals) to reduce the number of subchannels which need to be turned off around the frequency ranges that are causing interference may result in portions of the transmission exceeding a specified power spectral density mask. This can make the communications system non-compliant to technical and regulatory specifications and possibly prevent its sale and use.